This invention relates to an electron microscope and, more particularly, to an electron microscope adapted to prevent exposure due to X rays generated by scattered electrons in a camera compartment of the microscope.
In general, an electron microscope employs a mechanical shutter to perform photography. The construction of a conventional electron microscope using such a mechanical shutter is illustrated in FIG. 3. Shown in FIG. 3 are an electronic microscope 1, a camera compartment 2, an electron gun 3, an electron beam 4, a condenser lens 5, a specimen 6, an objective lens 7, a magnifying lens 8, a shutter 9, a recording medium transport means 10, a recording medium 11, a feeding magazine 12, a receiving magazine 13, unexposed recording medium 14 and exposed recording medum 15.
In the arrangement of FIG. 3, the shutter 9 and camera compartment 2 disposed at the lower part of the electron microscope 1 construct a photographic unit. Housed within the camera compartment 2 are the feed magazine 12 for accommodating the unexposed recording medium 14, the receiving magazine 13 for accommodatng the exposed recording medium 15, and the conveyance mechanism 10 for taking out the recording medium 11 from the feed magazine 12 and conveying it to a photographing position, and for subsequently conveying the recording medium 11 from the image formation plane to the receiving magazine 13.
In the electron microscope 1 thus constructed, the electron beam 4 from the electron gun 3 is condensed by the condenser lens 5 to irradiate the specimen 6, the electron image of which is formed by the objective lens 7. To record the electron image, the electron image is projected upon the recording medium 11, which has been supplied from the feed magazine 12, by the magnifying lens 8 for a period of time determined by the shutter 9, after which the recording medium is received by the receiving magazine 13.
Immediately before and after the electron image is recorded in the electron microscope of FIG. 3, the electron beam bombards the shutter 9, as a result of which X rays are produced. Also, during exposure, electrons are scattered within the camera compartment 2 and generate X rays. In the conventional electron microscope which uses photographic film as the recording medium 11, the photographic film has little sensitivity to X rays and almost no deposit due to X rays occurs. This makes it possible to record solely the electron image without any particular problems.
There has recently been proposed a new electron microscope system in which a two-dimensional sensor such as a stimulable phosphor sheet for storing irradiating electron beam energy is exposed to an electron beam passing through a specimen in a vacuum to have the electron beam energy stored in the two-dimensional sensor, the two-dimensional sensor is then exposed to light or heat to release the stored energy as light emission, the emitted light is photoelectrically detected to obtain an image signal, and the image of the electron beam which has passed through the specimen is reproduced by using the image signal. For example, see the specifications of U.S. patent application Ser. No. 754,996 and U.S. Pat. No. 4,651,220.
The two-dimensional sensor comprises a material which, when exposed to an electron beam, temporarily stores at least some of the electron beam energy. Then, when the material so exposed is subjected to stimulation from an external source, at least some of the stored energy is released in a detectable form as light, electricity, sound or the like. An example of a material particularly suitable for use as this two-dimensional sensor is a stimulable phosphor sheet as disclosed, for example, in Japanese Patent Application Laid-Open (KOKAI) Nos. 55-163472, 56-11395, and U.S. Pat. Nos. 4,258,264, 4,276,473, 4,387,428. More specifically, when certain kinds of phosphors are exposed to radiation such as an electron beam, they store a part of the energy of the radiation. When the phosphor which has been exposed to radiation is subsequently exposed to stimulating rays such as visible light, the phosphor exhibits light emission (fluorescence) in proportion to the stored energy. A phosphor exhibiting such properties is referred to as a stimulable phosphor. By "stimulable phosphor sheet" is meant a sheet-like recording medium comprising the aforesaid stimulable phosphor. In general, the stimulable phosphor sheet is composed of a substrate and a stimulable phosphor layer overlaid on the substrate. The stimulable phosphor layer comprises an appropriate binder and the stimulable phosphor dispersed therein, or solely the stimulable phosphor formed by vapor deposition. If the stimulable phosphor layer is self-supporting, the stimulable phosphor layer can be itself form the stimulable phosphor sheet. An example of a fluorescing phosphor for forming a stimulable phosphor layer sheet is described in detail in the aforementioned U.S. Pat. No. 4,651,220.
As the two-dimensional sensor, it is also possible to use a thermal phosphor sheet as disclosed, for example, in Japanese Patent Application Laid-Open Nos. 55-47719 and 55-47720. The thermal phosphor sheet is a sheet-line recording material primarily comprising a phosphor (thermal phosphor) which releases the stored radiation energy as thermal fluorescence mainly by the effect of heat.
In the above-described electron microscope system, the electron microscope image is stored in the two-dimensional sensor such as the stimulable phosphor sheet. This makes it possible to record the electron microscope image with high sensitivity. As a result, the amount of electronic beam exposure can be reduced to minimize damage to the specimen. In addition, it is easy to subject the electron microscope image in this system to such image processing as gradation processing and frequency response enhancement processing. Also, by applying the electric signal to a computer, it becomes possible to execute diffraction pattern processing, reconstruction of a three-dimensional image and image analysis for converting the image into binary values much more simply and rapidly in comparison with the prior art.
A problem that arises with the two aforesaid two-dimensional sensor is that since it is generally highly sensitive to X rays, the sensor is exposed by the penetrating X rays produced at the shutter 9, as a result of which the electron image is doubled. This causes a deterioration in the photographic image so that a clear image cannot be obtained.
Also, when the shutter 9 is retracted to expose the two-dimensional sensor to the electron beam, the unexposed or exposed two-dimensional sensor is exposed by the X rays generated by the scattered electrons in the camera compartment, thereby making it impossible to obtain a clear electron image.